In an existing heads up display, information is projected onto a specially treated portion of the windshield and reflected into the eyes of the driver. An important component of a head-up display system is known as the combiner. The combiner is positioned forward of the driver and extends partly across his or her view of the real world scene. It is usually either on the interior surface of or laminated inside of the windshield. It is constructed to transmit light from the real world scene ahead of the vehicle and to reflect light information of one or more particular wavelengths propagating from a source within the vehicle. The information is projected onto the combiner using suitable optical elements. The light rays reflected by the combiner are typically collimated to present an image of the information focused at optical infinity permitting the driver to simultaneously view the real world scene and the displayed information without changing eye focus.
Some combiners are simply semi-reflecting mirrors while a particularly effective combiner can be constructed using a hologram or a holographic optical element. In a currently used heads up display in motor vehicles, the motorist views the forward outside real world scene through the windshield. Information pertaining to the operational status of the vehicle is displayed on a heads up display system providing vehicle information, such as fuel supply and vehicle speed, positioned within the motorist's field of view through the windshield thereby permitting the motorist to safely maintain eye contact with the real world scene while simultaneously viewing the display of information.